1. RF Strategy & the MuCool Test Area Alan BrossMAP REVIEW 24-26 August, 20101 Meeting the RF in Magnetic Field Challenge

2. Outline – The “RF Challenge” – Science of RF Breakdowns – Current Program (Where we are) Derun Li’s talk (next) will discuss where we are going As will Katsuya Yonehara – The MuCool Test Area – Summary Alan BrossMAP REVIEW 24-26 August, 20102

3. Goals of This Talk We have a well defined and measured experimental program There are extensive scientific underpinnings for the program The experimental effort is supported by detailed simulation work which is predictive Alan BrossMAP REVIEW 24-26 August, 20103

4. Just to Review Remember, from Long Ago – (Yesterday) Alan BrossMAP REVIEW 24-26 August, 20104

5. Normal Conducting RF R&D Issues and Present Status Muon bunching, phase rotation and cooling requires Normal Conducting RF (NCRF) that can operate at high gradient within a magnetic field strength of up to approximately 6T – Required gradients (15-18MV/m) easily obtainable in absence of magnetic field Alan BrossMAP REVIEW 24-26 August, 20105 But

6. The RF Challenge Significant degradation in maximum stable operating gradient with applied B field Alan BrossMAP REVIEW 24-26 August, 20106 805 MHz RF Pillbox data – Curved Be windows – E parallel B – Electron current/arcs focused by B Degradation also observed with 201 MHz cavity – Qualitatively, quite different

7. 805 Pillbox Post-Mortem Alan BrossMAP REVIEW 24-26 August, 20107 Significant damage observed – Iris – RF coupler – Button holder However – No damage to Be window

8. 805 MHz Imaging Alan BrossMAP REVIEW 24-26 August, 20108 8 B Hot Spot Arc forms Cavity Energy W=1/2 CV 2  1-5 joule All goes into melting Cu Surface Field Enhancement Initiates the event & B focuses the e - current which causes damage

9. 201 MHz Cavity Test Treating NCRF cavities with SCRF processes The 201 MHz Cavity – Achieved 21 MV/m – Design gradient – 16MV/m – At 0.75T reached 10-12 MV/m However, No observed damage! Alan Bross9MAP REVIEW 24-26 August, 2010

10. 201 MHz Prototype Alan BrossMAP REVIEW 24-26 August, 201010 Note: Stored energy available to sparks  100J

11. RF Breakdowns Are not all equal – NCRF conditioning (B=0), process allows for higher gradient operation (“conditioning”) – NCRF (B  0), process can cause damage and require re-conditioning at lower gradient in order to reach the same gradient attainable before breakdown Alan BrossMAP REVIEW 24-26 August, 201011

12. The Science of RF Breakdown Vacuum In recent years, we have learned a great deal about the Science of RF Breakdown – Vacuum Arcs An explanation of the formation of high  asperities – Surface Field Enhancements Predictions of very high surface fields in arcs, consistent with measurements. An explanation of the microstructure in arc pits Preliminary results of arcs in static B fields. Comparison with studies of unipolar arcs. Calculations of sputtering and erosion rates. – Effects due to magnetically focused Field Emission Including studies with SCRF Alan BrossMAP REVIEW 24-26 August, 201012

13. The Science of RF Breakdown II Vacuum Alan BrossMAP REVIEW 24-26 August, 201013 Norem et al., 2001-2010 Workshop on Uni-polar Arcs, ANL, Jan. 2010 Breakdown Physics Workshop, CERN May 2010

14. The Science of RF Breakdown III Vacuum Advanced Simulation Code – OOPIC & VORPAL: Kevin Paul, Tech-X Alan BrossMAP REVIEW 24-26 August, 201014 New particles added (lost removed) {x α, v α } Particles accelerated by the fields {v' α } Particles moved based on new velocity {x' α } Currents “deposited” on the grid {J i } One Time Step Collisions and interactions computed New fields computed from old fields {E' i, B' i } This is where all the interesting physics for RF breakdown takes place!!!

15. The Science of RF Breakdown IV Vacuum Alan BrossMAP REVIEW 24-26 August, 201015

16. The Science of RF Breakdown V Vacuum Alan BrossMAP REVIEW 24-26 August, 201016 Dazhang Huang (IIT), Particle Studio Simulation 0 0.1 T 0.25 T 0.5 T

17. The Science of RF Breakdown VI Vacuum Numerical studies at BNL and SLAC (in collaboration) using Omega-3P and Track-3P codes, –Cavity with flat windows: 5 MV/m on axis; 2-T uniform external magnetic field; scan of a few points from one cavity side E field contour Trajectories without external B field Trajectories with external B = 2-T field

18. The Science of RF Breakdown Gas RF cavities filled with High-Pressure H 2 – Paschen’s Law Alan BrossMAP REVIEW 24-26 August, 201018 Rolland Johnson Shelter Island 2002

19. The Science of RF Breakdown II Gas Alan BrossMAP REVIEW 24-26 August, 201019 Gas Filled Cavities No focusing of electron avalanche + Electron Avalanche H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 V rf = V o Sin[  t] Cavity Energy W=1/2 C V 2  1 joule  Heats gas Collision frequency >> cyclotron frequency B has no effect ! Done? Note: 40MV/m & 100 Atm E/p  5

20. The Science of RF Breakdown III Gas However we want to operate with up to 10 13 muons/pulse – Beam-impact ionization + Ionization by secondary e - Alan BrossMAP REVIEW 24-26 August, 201020  + H 2   + H 2 + + e - e - + H 2  H 2 + + 2e - Most electrons (>90%) are quickly thermalized inside the cavity by elastic and inelastic collisions, and drift with RF until annihilated by recombination or attachment

21. The Science of RF Breakdown IV Gas Alan BrossMAP REVIEW 24-26 August, 201021 1. Rapid decay of pickup signal according to the ionization rate 2. Saturation level and recovery rate determined by the recombination rate Gray line: normal signal without beam Solution:? Electron “getter” Electro-neg. Gas see K. Yonehara’s talk

22. Science of RF Breakdown Summary Although the study of breakdown in RF cavities is an active (& continuing) field of research, and academic study of RF breakdown is not MAP’s mission We know: – Without surface field enhancements, there is no field emission – Without field emission, the events that lead to the damaging breakdowns (B  0 will not occur) So: – Eliminate ( ameliorate ) the surface field enhancements – Or mitigate the damaging effects to the cavity from the resulting events Alan BrossMAP REVIEW 24-26 August, 201022

23. NCRF Program R&D Strategy  Technology Assessment (continuation of existing multi- pronged program) – Surface Processing Reduce (eliminate?) surface field enhancements, field emission – SCRF processing techniques » Electro-polishing (smooth by removing) + HP H 2 O rinse – More advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level) – Materials studies: Use base materials that are more robust to the focusing effects of the magnetic field Cavity bodies made from Be or possibly Mo – Magnetic Insulation Inhibit focusing due to applied B – High-Pressure Gas-filled (H 2 ) cavities Alan BrossMAP REVIEW 24-26 August, 201023 Vacuum

24. 201 MHz Cavity Test Treating NCRF cavities with SCRF processes 21 MV/m Gradient Achieved (Design – 16MV/m) – Limited by available RF power (4.5 MW) 24MAP REVIEW 24-26 August, 2010Alan Bross

25. 201 MHz Cavity Running Summary I (B=0) Design Gradient Limited by RF Power 25MAP REVIEW 24-26 August, 2010Alan Bross

26. 201 MHz Cavity Running Summary II (B>0) 26MAP REVIEW 24-26 August, 2010Alan Bross

27. 201 MHz Cavity B Field Tests Summary Sparking @ B=0 did condition the 201 cavity Sparking @ B  0 causes damage (B relatively low) Although we “broke” the 201, it did re-condition @ B=0. But upon inspection of the cavity – No observed damage SCRF processing techniques help 27MAP REVIEW 24-26 August, 2010Alan Bross

28. Advanced Processing Concept Atomic Layer Deposition (ALD) Field enhancements in RF cavities root cause of surface breakdown – Due to high local electric fields that exist at surface asperities (field enhancements) – Cover asperities with a conformal coatings applied with Atomic Layer Deposition (ALD) – Has been applied to SCFR Alan BrossMAP REVIEW 24-26 August, 201028 1 example of ALD processing of 1.3GHz SCRF cell

29. NCRF R&D Program  Potential paths towards a solution: Phase I: Technology Assessment (continuation of existing program) Multi- pronged approach: – Surface Processing Reduce (eliminate?) surface field enhancements, dark current – SCRF processing techniques » Electro-polishing (smooth by removing) + HP H 2 O rinse – More advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level) – Materials studies: Use base materials that are more robust to the focusing effects of the magnetic field Cavity bodies made from Be or possibly Mo – Magnetic Insulation – High-Pressure Gas-filled (H 2 ) cavities Alan BrossMAP REVIEW 24-26 August, 201029 Vacuum

30. Material Studies “Button” system in pillbox cavity designed for easy replacement of test materials – Tested so far: TiN-coated Cu & Mo, bare Mo and W – Results to date indicate that Mo can improve performance at a given B field by somewhat more than 50% – 16.5MV/m  26MV/m Alan BrossMAP REVIEW 24-26 August, 201030 Molybdenum buttons (1.7x field enhancement factor on button surface)

31. Button RF Cell Data Summary Alan Bross31MAP REVIEW 24-26 August, 2010

32. NCRF R&D Program  Potential paths towards a solution: Phase I: Technology Assessment (continuation of existing program) Multi- pronged approach: – Materials studies: Use base materials that are more robust to the focusing effects of the magnetic field Cavity bodies made from Be or possibly Mo – Surface Processing Reduce (eliminate?) surface field enhancements, dark current – SCRF processing techniques » Electro-polishing (smooth by removing) + HP H 2 O rinse – More advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level) – Magnetic Insulation – High-Pressure Gas-filled (H 2 ) cavities Alan BrossMAP REVIEW 24-26 August, 201032 Vacuum

33. Magnetic Insulation Although lattices that employ magnetic insulation have drawbacks with respect to the required RF power, we are studying the concept using a newly completed 805 MHz box cavity Conceptual Design 33MAP REVIEW 24-26 August, 2010Alan Bross

34. MAP REVIEW 24-26 August, 201034

35. Box Cavity in Solenoid 35MAP REVIEW 24-26 August, 2010Alan Bross Max angle w/r to horizontal  12 0 – E at 78 o w/r to B Max Gradient (B=0) – 40MV/m

36. NCRF R&D Program  Potential paths towards a solution: Phase I: Technology Assessment (continuation of existing program) Multi- pronged approach: – Materials studies: Use base materials that are more robust to the focusing effects of the magnetic field Cavity bodies made from Be or possibly Mo – Surface Processing Reduce (eliminate?) surface field enhancements, dark current – SCRF processing techniques » Electro-polishing (smooth by removing) + HP H 2 O rinse – More advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level) – Magnetic Insulation – High-Pressure Gas-filled (H 2 ) cavities Alan BrossMAP REVIEW 24-26 August, 201036 Vacuum

37. High Pressure H 2 Filled Cavity Work with Muons Inc. High Pressure Test Cell Study breakdown properties of materials in H 2 gas Operation in B field – No degradation in M.S.O.G. up to  3.5T Next Test – Repeat with beam No Difference B=0 & B=3T 37MAP REVIEW 24-26 August, 2010Alan Bross Well beyond gradient requirement for HCC

38. High Pressure H 2 Filled Cavity Results In Surface Breakdown Regime Alan BrossMAP REVIEW 24-26 August, 201038 Pit distribution fit to E max (ANSYS)  Fowler-Nordheim

39. RF Test Facility MuCool Test Area (MTA) – RF power 201 MHz (5MW) 805 MHz (12 MW) – Class 100 clean room – 4T SC solenoid 250W LHe cryo-plant – Instrumentation Ion counters, scintillation counters, optical signal, spectrophotometer – 400 MeV p beam line Alan BrossMAP REVIEW 24-26 August, 201039

40. MTA Layout Alan BrossMAP REVIEW 24-26 August, 201040

41. MTA RF Alan BrossMAP REVIEW 24-26 August, 201041

42. MTA Cryo Valve Box & Transfer Line Alan BrossMAP REVIEW 24-26 August, 201042

43. MTA Hall – Clean Room 43MAP REVIEW 24-26 August, 2010Alan Bross

44. MTA Hall – Clean Room II Alan BrossMAP REVIEW 24-26 August, 201044 Goal for Clean room : Class 100 – Achieved better than Class 10 Even with 3 people inside: Class 40 Goal for Hall: Class 1000 – Achieved Class 500

45. MTA Instrumentation Alan BrossMAP REVIEW 24-26 August, 201045 Counters Optical Diagnostics DAQ RF Pickup X-ray Optical

46. Current MTA Beam Line Alan BrossMAP REVIEW 24-26 August, 201046

47. MTA Beam Line Status Beam Line Installation – Complete Beam Line commissioning to first beam stop. – Complete First pass @ Radiation assessment review – Complete First beam experiments by the end of the year The MTA is a World-Class Facility (& Unique): High-Power RF; High-Intensity Beam; H 2 handling, SC Magnet 47MAP REVIEW 24-26 August, 2010Alan Bross

48. Phase I RF Program (2 year) D. Li, Next Talk Complete tests on Magnetic Insulation – Second box test series with E  B – Box with orientation E  B Be materials tests – Button cavity test – Be wall cavity ALD coated button test – And with purpose RF cavity Beam tests of high pressure H 2 filled cavity 201 MHz tests in higher B field – Need new SC magnet - FY2012 Alan BrossMAP REVIEW 24-26 August, 201048

49. RF Strategy Down Selecting Down selection of RF cavities will be based on the outcome of the experimental studies. The successful cavity technology must work at an acceptable RF gradient (requirements are, of course, dependent on the position along the channel, i.e., phase rotation, bunching, initial cooling, final cooling, etc.) in a multi-tesla magnetic field. Engineering, fabrication, integration, and cost of the cavity and RF power must also be considered Alan BrossMAP REVIEW 24-26 August, 201049

50. Summary We have a comprehensive program aimed at developing a solution to the “RF Challenge” The experimental program is robust and scope-appropriate, is coupled with mature simulation efforts and is backed by a good understanding of the physics of breakdown/arcs in RF structures This multi-pronged approach gives us confidence that we can meet this challenge and minimize the risk to a future Muon Collider Alan BrossMAP REVIEW 24-26 August, 201050

51. Acknowledgments The ongoing effort on NCRF has a long list of contributors who have contributed to this presentation. I thank them all (in no particular order): Bob Palmer Jim Norem Alvin Tollestrup Moses Chung Bob Rimmer Derun Li Al Moretti Pierrick Hanlet Zubao Qian Dazhang Huang Katsuya Yonehara Yagmur Torun Milorad Popovic Diktys Stratakis Kevin Paul Steve Virostek Thomas Prolier Alan BrossMAP REVIEW 24-26 August, 201051

52. END Alan BrossMAP REVIEW 24-26 August, 201052

53. But it is not a Cliff Alan BrossMAP REVIEW 24-26 August, 201053  /p after initial (4D cooling)